Hadith: Gold for gold is usury unless they are exchanged on the spot; silver for silver is usury unless they are exchanged on the spot; wheat for wheat is usury unless they are exchanged on the spot; and barley for barley is usury unless they are exchanged on the spot

عن عمر بن الخطاب رضي الله عنه مرفوعاً: «الذهب بالذهب رِباً، إلا هَاءَ وَهَاءَ، والفضة بالفضة ربا، إلا هَاءَ وهَاءَ، والبُرُّ بالبُرِّ ربا، إلا هاء وهاء. والشعير بالشعير ربا، إلا هاء وهاء».
[صحيح] - [متفق عليه]

‘Umar ibn al-Khattāb (may Allah be pleased with him) reported that the Prophet (may Allah's peace and blessings be upon him) said: "Gold for gold is usury unless they are exchanged on the spot; silver for silver is usury unless they are exchanged on the spot; wheat for wheat is usury unless they are exchanged on the spot; and barley for barley is usury unless they are exchanged on the spot."
Sahih/Authentic. - [Al-Bukhari and Muslim]

Explanation

The Prophet (may Allah's peace and blessings be upon him) clarified in this Hadīth how to conduct a valid sale transaction in which the substance of exchange is susceptible to interest. The condition in selling gold for silver or silver for gold is that the exchange (Taqābud) is made in one place and time before the two parties depart, expressed by the translation "on the spot'. Otherwise, the transaction is invalid because this is considered a money exchange, and such an exchange must be made on the spot in order to be valid. Similarly, for the one who sells wheat for barley or vice versa, it is a condition that they are exchanged on the spot before the two parties depart because there is a potential cause or reason for prohibition, which is interest, if they parted before the exchange is made. If the two items are of the same kind, then it is a condition that they are exchanged on the spot and be of the same amount or value (Tamāthul / Musāwāh), such as gold for gold, even if the quality of gold is different. If they are of two different kinds but the reason for prohibition is the same, then it is a condition that they are exchanged on the spot, but it is not a condition that they are of the same amount or value, such as gold in exchange for coins. If the reason for prohibition is different or if the substance of exchange is not prone to usury, then there are no conditions for the transaction. Deferment of exchange and difference in amount or value is permissible.